.\"	$OpenBSD: getentropy.2,v 1.8 2015/01/31 00:20:12 schwarze Exp $
.\"
.\" Copyright (c) 2018 Conrad Meyer <cem@FreeBSD.org>
.\" Copyright (c) 2014 Theo de Raadt
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.Dd January 17, 2025
.Dt GETENTROPY 3
.Os
.Sh NAME
.Nm getentropy
.Nd get entropy
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn getentropy "void *buf" "size_t buflen"
.Sh DESCRIPTION
.Fn getentropy
fills a buffer with high-quality random data.
.Pp
The maximum
.Fa buflen
permitted is 256 bytes.
.Pp
If it does not produce an error,
.Fn getentropy
always provides the requested number of bytes of random data.
.Pp
Similar to reading from
.Pa /dev/urandom
just after boot,
.Fn getentropy
may block until the system has collected enough entropy to seed the CSPRNG.
.Sh IMPLEMENTATION NOTES
The
.Fn getentropy
function is implemented using
.Xr getrandom 2 .
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn getentropy
will succeed unless:
.Bl -tag -width Er
.It Bq Er EFAULT
The
.Fa buf
parameter points to an
invalid address.
.It Bq Er EINVAL
Too many bytes requested.
.El
.Sh SEE ALSO
.Xr getrandom 2 ,
.Xr arc4random 3 ,
.Xr random 4
.Sh STANDARDS
.Fn getentropy
conforms to
.St -p1003.1-2024 .
.Sh HISTORY
The
.Fn getentropy
function appeared in
.Ox 5.6 .
The
.Fx
libc compatibility shim first appeared in
.Fx 12.0 .
